swi4: clock taking 40% cpu?!?

Dan Nelson dnelson at allantgroup.com
Thu Dec 15 21:35:43 UTC 2011


In the last episode (Dec 15), Jeremy Chadwick said:
> On Thu, Dec 15, 2011 at 12:51:28PM -0800, Doug Barton wrote:
> > Web server under heavy'ish load (7 on a 2 cpu system) running
> > 8.2-RELEASE-p4 i386 I'm seeing this:
> > 
> > PID USERNAME PRI NICE   SIZE    RES STATE   C   TIME   WCPU COMMAND
> > 12  root     -32    -     0K   112K WAIT    0 129:01 39.99% {swi4: clock}
> > 
> > Any ideas why the clock should be taking so much cpu? HZ=100 if that
> > makes a difference ...
> 
> Could be wrong, but I believe this correlates with IRQ 4.  What does
> vmstat -i show for a total and rate for irq4 if you run it, wait a few
> seconds, then run it again?  Does the number greatly/rapidly increase?

That would be "irq4" in that case, though.  "swi4" is just a software
interrupt thread, and "clock" is the softclock callout handler.  There are
both KTR and DTrace logging functions in kern_timeout.c, so you could use
either one to get a handle on what's eating your CPU.  Busy-looping
"procstat -k 12" for a few seconds might get you some useful stacks, as
well.
 
> Shot in the dark here, but the only thing I can think of that might
> cause this is software being extremely aggressive with calls to things
> like gettimeofday(2) or clock_gettime(2).  Really not sure.  ntpd maybe
> (unlikely but possible)?  Sort of grasping at straws here.

-- 
	Dan Nelson
	dnelson at allantgroup.com


More information about the freebsd-stable mailing list